The continuous fermentation of a milk by lactic acid bacteria requires stirring of the milk so that unfermented milk can be added to and fermented milk drawn off from the fermentation tank. When the pH of such a stirred milk reaches a pH of 4.8-5.2, the milk casein micelles tend to agglomerate and to become concentrated until they become irreversibly coagulated. This coagulation phenomenon, called "syneresis", unfortunately destroys the consistency of the fermented milk (Veisseyre, Technologie du Lait, Ed. La Maison Rustique, Paris, 1975, I.S.B.N. 2-7066-0018-7, pages 427-438).
To overcome this disadvantage, a fermented milk may be prepared in a semicontinuous manner, by first preparing continuously a preculture of lactic acid bacteria having a pH greater than 5 so as to avoid the appearance of a complete syneresis, then secondly by inoculating a milk with this preculture and by subjecting it to batch fermentation without stirring up to a pH of about 4.5.
The continuous preparation of a preculture of lactic acid bacteria, as well as the use of this preculture in the context of the semicontinuous manufacture of a fermented milk is well known. By way of examples, there may be mentioned the processes described by Dries sen et al., (Biotechnology and Bioengineering, 29, 821-839, 1977), by Lelieved et al., (Bioengineering, 11, 39-40, 1976), by Lelieved (Process Biochemistry, 19, 112-113, 1984), by MacBean et al., (Biotechnology and Bioengineering, 21, 1517-1531, 1979), by Prevost et al., (Milchwissenschaft, 43, 622-719, 1988), by Reichart (Acta Alimentaria, 8, 373-381, 1979), by Schuler (DE2006894) and by Spreer et al., (Lebensmittel-biotechnologie, 7, 28-31, 1991).
During a continuous process for the preparation of a fermented milk, it is also possible to partially prevent the complete formation of a syneresis.
To this effect, U.S. Pat. No. 3,924,007 describes a continuous process in which, a preculture of lactic acid bacteria having a pH greater than 5 is first continuously prepared so as to avoid syneresis of the milk, and then secondly, this preculture is continuously fermented while it is being stirred in a manner such that a stirring gradient becomes established in the fermented milk, avoiding the appearance of a complete syneresis of the milk.
Such a stirring gradient has also been used by Van der Loo in the context of the continuous preparation of a fermented milk (Deutsche Milchwirtschaft, 29, 1199-1202, 1980). In this process, milk and a preculture of lactic acid bacteria are added at the top of a fermentation tank at pH 5.7, the fermented milk is subjected, in the tank, to a stirring gradient, and the fermented milk is drawn off from the bottom of the tank.
Unfortunately, these continuous processes for the preparation of a fermented milk are not completely satisfactory. Indeed, milk derived from a continuous fermentation never have a pH of less than 4.7, for fear of reinforcing syneresis of the milk. In some cases, these fermented milks are also subjected to shear forces such that the consistency of the milk becomes profoundly modified as a result.
The present invention aims to overcome the disadvantages of the prior art by providing a process which can be easily conducted.